qapi: Fix calc-dirty-rate example

The example shows {"command": ...}, which is wrong.  Fix it to
{"execute": ...}.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20220401082028.3583296-1-armbru@redhat.com>
Reviewed-by: Victor Toso <victortoso@redhat.com>
1 file changed